1. Field of the Invention
The present invention is concerned with a production process and apparatus for producing cement clinker, and may be used in the industry of building materials.
2. Prior Art
It is a common knowledge that the cost of the cement depends to a great extent on the specific consumption of energy consumed in the production of the cement clinker. It is also a well known fact that the production of cement clinker is associated with a considerable non-productive consumption of heat and, hence, with an extra consumption of fuel. Many attempts have been made to reduce a non-productive consumption of fuel by improving equipment and production processes. These attempts, however, led, in general, to more complex equipment having a greater weight and size, and the gains obtained due to a higher factor of the utilization of energy are brought to nothing by a higher cost of the equipment. A great number of patents granted in the last 10-15 years for improvements in the above apparatus and methods shows that this problem remains to be adequately solved yet.
A conventional process of producing cement clinker (cf., for example, R. Fogel, Silikattechnik No. 9, 1966) comprises preheating powdered cement raw material and treating the same in a rotary kiln wherein this material is decarbonized in a stirred bed and burned into clinker as it is moving towards the discharge side of the kiln.
It is clear that the particles of the material being burned, which are on/or close to the surface of the stirred bed are exposed to the most intensive heating, and therefore heating the layer of material through its whole depth requires a considerable amount of energy, thereby causing increased consumption of fuel. The combustion gases discharged from the kiln have a temperature in the order of 900.degree.-1200.degree. C., and therefore only a relatively small portion of their heat can be utilized. In addition, the intensity of the decarbonization within a stirred bed is low, which affects the efficiency of the apparatus.
It has been tried to overcome this disadvantage by separating the process of decarbonization from burning, which has been embodied in a method of producing cement clinker (cf. French patent specification No. 2,235,890), wherein powdered cement raw material is preheated to a temperature of 800.degree.-900.degree. C. and then introduced into a decarbonizer in which powdered cement raw material is decarbonized in a suspended state by burning a portion of the fuel. The other portion of the fuel is burned within the kiln into which is also introduced the decarbonized material. As this material moves from the charge to the discharge end of the kiln it is burned in a stirred bed into granules. In carrying out this method 40-50% of the fuel is burned in the decarbonizer, while the rest thereof is fed into the kiln. Preheating cement raw material powder is effected by the combustion gases withdrawn from the decarbonizer and the kiln. Decarbonization of the material being in a suspended state proceeds far much intensively than in a stirred bed, and therefore the efficiency of the apparatus for carrying out this method is considerably higher. However, using the above method is associated with higher losses of heat, caused by the heat radiation through the walls of the decarbonizer. In addition, separately heating the powdered raw material for the purpose of decarbonization, and burning the same requires more fuel, and leads to the increase of the size, weight, and hence cost of the equipment.
There is also known an apparatus wherein the above disadvantage has been mitigated by combining the decarbonization and burning in one process (cf. FRG Pat. No. 2,061,980). According to this method the powdered cement raw material is preheated to a temperature of 800.degree.-900.degree., decarbonized in a suspended state and burned. Decarbonizing powdered cement raw material is done by injecting the latter in an air flow into the kiln through the charge end thereof. The fuel is fed simultaneously through the same end. The burning fuel forms a flow of combustion gases. Burning the carbonized material into cement clinker is also effected in a suspended state. After being discharged from the kiln the suspension of the cement clinker thus obtained is separated into a solid and a gaseous phases. Being separated the combustion gases are further used for preheating cement raw material.
An apparatus for carrying out the above method comprisses cyclone heat exchangers connected in series with the charge end of a rotary kiln. Coaxially introduced into the kiln through the charge end thereof is an ejector connecting the said cyclone heat exchangers with the inner space of the kiln. The ejector includes a burner, and is connected with a source of air. At the discharge end of the kiln is disposed a separator-precipitator made in the form of a cyclone precipitator communicated with the cyclone heat exchanger for partially utilizing the heat of combustion gases.
Elimination of the separate decarbonizer permits the heat losses caused by radiation to be decreased, and the size and weight of the apparatus to be reduced.
Nevertheless, the above method have not found a wide application for a number of reasons. Thus, burning the decarbonized material being in suspension does not provide for a full burning of the whole mass of the material being treated, thereby affecting the quality of the resulting cement clinker. To some extent this could be remedied by increasing the amount of fuel being burned. However, this would inevitably entail a considerable increase in the temperature of the clinker within the precipitator. As a result, a layer of clinker is rapidly deposited on the walls of the precipitator, which makes it necessary to periodically stop the operation for cleaning. In addition, for the utilization of heat of the gaseous combustion products having a temperature in the order of 1400.degree. C. the apparatus must be provided with a multistage system of heat exchangers at least five cyclone heat exchangers. The more heat exchangers, the larger surface of heat transfer, and hence higher irrepairable losses of heat. At the same time the presence of these cyclone heat exchangers increases a flow resistance and required a more powerful exhaust fan.
Increasing the length of the rotary kiln in order to prolong the time of burning is not expedient from the economical point of view.